Why can running feel so bad? (15.10.2009)

I just came back from a 40 minute run. I have no idea what is the reason why all aerobic exercise fees so bad. I started to get cramps in both calfs after 2 km. I also felt like my lungs would give in and I just couldn't run for more than 500 m at a time. I somehow need to pace myself better in order to be able to run without breaks.
I ran for 6,18 km as you can see from the RunKeeper data at http://bit.ly/fh4AX. Total calories burned was 741 and TE 3.9. Average heart rate was 153.

I need to try to run more even though it doesn't feel to good. I still want to run the Cooper test before winter. If all goes well I wish I will do the test in little over a week.

I think I'll go swimming tomorrow after work and then maybe for a gym workout later at night.